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| | | #REDIRECT [[Radio acoustic sounding system]] |
| #REDIRECT [[radio acoustic sounding system]] | |
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| == radio acoustic sounding system ==
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| <div class="definition"><div class="short_definition">(Abbreviated RASS.) A ground-based [[remote sensing]] method that combines [[radar]] and acoustic techniques to determine the [[vertical profile]] of [[virtual temperature]], often used in conjunction with [[wind profilers]].</div><br/> <div class="paragraph">With the RASS technique, [[sound waves]] generated by an acoustic source located near the radar produce perturbations in the atmospheric [[refractive index]]. The perturbations propagate upward in the air at the local [[speed of sound]] and serve as a [[target]] for the radar. When the acoustic [[wavelength]] is matched to half the radar wavelength (the [[Bragg scattering]] condition), a [[resonance]] condition develops that leads to the production of a detectable [[radar echo]]. The [[Doppler shift]] of this echo is a measure of the sum of the [[speed of sound]] and any vertical air motion in the [[radar volume]]. Early work with RASS either ignored the vertical air motion or used time averaging to reduce its effect. Some modern RASS processors have the ability to correct for vertical air motion. The local speed of sound is related to the virtual temperature by <div class="display-formula"><blockquote>[[File:ams2001glos-Re11.gif|link=|center|ams2001glos-Re11]]</blockquote></div> where ''C''<sub>''s''</sub> (m s<sup>-1</sup>) is the speed of sound, ''R'' is the [[gas constant]] of air, γ (= ''c''<sub>''p''</sub>/''c''<sub>''v''</sub>) is the ratio of the [[specific heats]] of air, and ''T''<sub>''v''</sub> is the virtual temperature in degrees Kelvin. Using this relation, the [[vertical profile]] of sound speed can be converted to a profile of virtual temperature. The RASS technique depends on close matching of the acoustic wavelength to half the radar wavelength (called Bragg matching). To ensure that Bragg matching occurs throughout the [[range]] of temperature expected in the vertical profile, the acoustic source is swept in [[frequency]] over an interval broad enough to contain all the necessary spatial wavelengths. Bragg matching also requires that different acoustic frequency bands be used for different radar frequencies. Acoustic frequencies range from around 100 Hz for 50-MHz radars up to 2 kHz for radars operating around 1 GHz. Because acoustic [[attenuation]] in the [[atmosphere]] is strongly dependent on frequency, the height coverage or reach of RASS varies greatly with radar frequency. Typical small UHF [[boundary layer radars]] can measure RASS temperature profiles to about 1.5 km, whereas large VHF [[MST radars]] can measure the [[temperature]] up to 10 km or higher. <br/>''See also'' [[electromagnetic acoustic probe]].</div><br/> </div>
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| [[Category:Terms_R]]
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